RNA-directed DNA methylation is involved in regulating photoperiod-sensitive male sterility in rice

Photoperiod-sensitive male sterility (PSMS) is a valuable germplasm for hybrid rice breeding. Recently, we cloned pms3, a locus controlling PSMS, which encodes a long non-coding RNA called LDMAR required for normal male fertility of the rice plant under long-day conditions. Increased methylation in the promoter of LDMAR in the PSMS rice (Nongken 58S) relative to the wild-type (Nongken 58) reduced expression of LDMAR leading to male sterility under long-day conditions. In this study, we identified a siRNA named Psi-LDMAR in the LDMAR promoter region that was more abundant in Nongken 58S than in Nongken 58. We showed that Psi-LDMAR was likely derived from AK111270, a transcript obtained from the sense strand of the LDMAR promoter with its 3'-end having a 110-base overlap with the 5'-end of LDMAR. Overexpressing AK111270 in Nongken 58S greatly enriched Psi-LDMAR, which induced RNA-directed DNA methylation in the LDMAR promoter and repressed the expression of LDMAR. Reduction of LDMAR in Nongken 58S changed the critical day length for fertility recovery and delayed the fertility recovery under short-day conditions. This result added to our understanding of the molecular mechanism for PSMS.